1. Field of the Invention
The present invention relates to intruder alarm systems of the type used to detect mechanical vibrations in a security fence or the like and to generate an alarm signal accordingly. A serious problem arises in such systems owing to the difficulty of discriminating between vibrations arising from interference with the security fence by a would-be intruder and vibrations arising naturally e.g. from wind.
2. Description of Related Art
Various discriminator circuits have been designed in order to distinguish between spurious and genuine intruder signals. One such circuit is disclosed in U.K. Pat. No. 2,045,494B and has been found to be effective. However there exists a continuing need for improved signal processing techniques based on an improved knowledge of the signal, so as to discriminate more reliably between processed signal patterns arising from intrusion attempts and those arising from environmental sources such as wind, birds and other small animals. In particular it has been difficult to detect intrusion attempts when high winds act on the security fence.